1. Field of the Invention
The present invention relates to pressure operated drives and, more particularly, to control of the drive in the event of system malfunction.
2. Description of the Related Art
Pressure operated drives, especially pneumatic drives, are well known and often used to actuate translatory or rotatory controlling elements or valves in process engineering. Pneumatic drives are well suited for use in systems subject to explosion protection standards as no protective measures are needed for these drives. When such drives contain electric or electronic components they are normally designed in accordance with known and accepted intrinsically safe ignition protection standards. Many pressure operated drives employ a nozzle/flapper system as is known from German laid open document No. DE 41 42 269 for supplying and controlling flow of a pressure medium to the drive.
Pneumatic servo drives recently have been designed to assume a fail safe position in the event of system failure or malfunction to prevent endangerment to the entire system. These drives include deventilation valves to ensure a defined state of both the drive and controlling element in such events. During normal operation the drive is directly controlled by a preset pressure valve or with the help of a positioner, but upon occurrence of an emergency the deventilation valve acts to open the drive volume of the drive to the atmosphere resulting in deventilation and a spring moves the pneumatic drive into a safe position in which damage to the drive during such operation is prevented. The output of the deventilation valve is designed so that its effect overrides that of the components regulating the position of the drive during normal operation. This deventilation valve is normally designed as a magnetic valve and controlled by a separate signal from a measurement station.
At the same time as the deventilation valve opens the drive volume the controlling flow produces an actuating force, via the electromagnetic interaction between the measurement station and the deventilation valve, which moves the pneumatic valve cone or valve slide. The design of this system has the problem of needing high electric driver wattages of approximately 1 watt. These high wattages are not compatible with intrinsically safe systems of ignition protection according to standard DIN EP 50020 as is usually desired for such pneumatic servo drives. In order to meet the applicable explosion standards for safe operation of such systems in their normal operating environment, other types of ignition protection systems, for example, flameproof housings must be used which greatly increase the cost of the device.
Such deventilation valves are known from the brochure "SAMSOMATIC, Magnetic Valve Technology" by SAMSON AG. The deventilation valves described in this brochure are mounted on the valves by additional casing on the pressure medium side. However, the attachment of these valves and the embodiment needed to meet the explosion protection standards cause the system to be very expensive.
It is thus desirable to provide a pneumatic servo drive which is able to implement an inexpensive rapid deventilation system in an intrinsically safe ignition protection system.